1. Field of the Invention
The subject invention generally pertains to centrifugal blowers whose impellers have forward curved fan blades and more specifically to a fan inlet shroud for such a blower.
2. Description of Related Art
Centrifugal blowers, such as those disclosed in U.S. Pat. Nos. 5,570,996; 5,558,499; 3,627,440; 3,307,776; 3,217,976; 2,981,461; 2,951,630; 2,798,658; 2,727,680; 3,221,983 and 1,862,523, are fans that draw air in an axial direction and discharge the air radially relative to the rotational axis of the blower's impeller. To create such a flow pattern, centrifugal blowers often include a scroll shaped housing or volute that contains an impeller of a particular design.
There are countless centrifugal impeller designs such as impellers with backward inclined fan blades, forward curve fan blades, radial or flat paddlewheel blades, airfoil or streamlined blades, and various combinations thereof. The present invention pertains to forward curved blades whose distinct operating characteristics make them suitable for applications that are quite different than that of backward inclined blades.
Backward inclined blades are generally for high pressure, low volume applications. Backward inclined blades, or at least their trailing edges, lean away from their direction of travel. Since volume generally increases with the axial width of the impeller and pressure generally increases with impeller diameter, impellers with backward inclined blades tend to have a relatively large outer diameter as compared to their narrow fan blade length (i.e., relatively large ratio of impeller diameter to axial blade length). Backward inclined blades are more sparsely distributed circumferentially around the impeller (lower solidity). The blades typically have a longer chord length, which is the distance between the blade's leading edge (at the impeller's inner diameter) and the blade's trailing edge (at the impeller's outer diameter). Impellers with backward inclined blades are generally quieter, faster, and more efficient than their forward curved counterparts.
To achieve such performance, minimizing the angle of attack of the incoming air is critically important with backward inclined blades, so the blower housing preferably includes an inlet shroud having a curved contour that accurately directs the incoming air in a favorable direction into the leading edge of the blade.
With forward curved blades, the airflow pattern across the blade is dramatically different. The blade edge near the impeller's outer diameter leans into rather than away from the blade's direction of travel. Thus the aerodynamic requirements of an inlet shroud can vary greatly between forward and backward inclined blades. Simply replacing the inlet shroud of any forward curved blade with that of a backward inclined blade will not necessarily provide desirable results.
In many respects, the desired results of a blower with forward curved blades are in stark contrast with that of blowers with backward inclined blades. Forward curved blades are generally for low pressure, high volume applications, such as moving air in a HVAC application. Impellers with forward curved blades have generally higher solidity, run at lower speeds, and usually operate at lower efficiency than impellers with backward inclined blades. Lower efficiency not only increases operating costs, but can also create high energy, low frequency noise, which is particularly difficult to suppress.
Since impellers with forward curved blades are often used in HVAC air moving applications where people are present, minimizing low frequency noise is important. Thus, there is an ongoing need for an ever quieter forward curved fan.